For this a hydraulic pump is driven by the drive motor, e.g. a combustion engine in particular a diesel engine and provides the hydraulic motor with hydraulic power in form of supplying a hydraulic fluid flow under high pressure. The hydraulic motor converts the hydraulic power supplied by the hydraulic pump into mechanical power for the propulsion of a consumer. In case of a hydrostatic transmission the hydraulic motor drives the driving wheels of a working machine or a vehicle. In coasting mode, i.e. when the vehicle wheels drive the hydraulic motor, the flow of power runs from the hydraulic motor to the hydraulic pump, whereas the directions of rotation and conveyance remain the same as well as the direction of flow in the hydraulic lines, and only the high pressure line switches with the low pressure line and vice versa.
In such hydrostatic drives closed hydraulic circuits are used preferably, whereby both hydrostatic driving mechanisms—the hydraulic pump and the hydraulic motor—are connected by hydraulic lines, being according to the drive direction of the hydraulic pump a supply or a return line for hydraulic fluid. Further, depending on the operation mode—acceleration mode or coasting mode—every line can be a low pressure line or a high pressure line. Hence, each of the hydraulic lines connecting both hydrostatic driving mechanisms with each other can be i) a supply line with hydraulic fluid under high pressure or ii) a return line with hydraulic fluid under low pressure as well as iii) a supply line for fluid with low pressure or iv) a return line with hydraulic fluid under high pressure (coasting mode).
Particularly, in coasting mode, when the hydraulic motor provided for the propulsion, works as a pump and the hydraulic pump has to receive the hydraulic fluid conveyed by the hydraulic motor, a breaking, coasting or drag torque of the driving motor is used for decelerating the hydrostatic drive. In such a coasting mode, e.g. by downhill drive of a vehicle or a working machine, the hydraulic motor drives the pump which in turn drives the driving motor. As the hydraulic pump working in coasting mode as a hydraulic motor as well as the drive motor only bears limited rotational speed, in order to avoid damages at the respective driving mechanism a speed limitation has to be provided.
For this reason, according to DE 199 30 997 B4 a throttle valve was proposed to be introduced into the return line and which comes into effect if the hydraulic fluid flowing from the hydraulic motor to the displacement pump shows an increased pressure. If the pressure in the return line passes a given threshold value, the throttle comes into effect and the pressure being conveyed to the hydraulic pump by the return line is limited to a fixedly and unchangeable set support pressure during the further coasting mode of the work machine or the hydrostatic drive. The excess of hydraulic power introduced by the hydraulic motor into the return line is, according to DE 199 30 997 B4 transformed in heat energy—as customary according to the state of the art.
In DE 10 2004 030 045 B3 a throttle being controlled by a pressure relief valve is foreseen for avoiding overspeed of the driving motor. The control pressure for controlling the throttle valve is generated by a pilot pressure taken from the return line. The maximum of the control pressure is given by a pilot pressure relieve valve fixedly and unchangeable. The pilot pressure relieve valve opens by exceeding the permissible pilot pressure in the pilot line, whereby the pilot line is fluidly connected with the return line. The pressure of the return line now moves the throttle valve by pressurizing the throttle valve spool against the lower pilot pressure into a position in which the flow rate cross-section is lowered. Thereby, the pressure in the return line and, hence, as well in the pilot line decreases. If the pressure in the pilot line falls below the threshold pressure of the pilot relieve valve, the pilot relieve valve closes again and the pressure in the return line increases the pilot pressure again, what in turn augments the flow rate cross-section by shifting the throttle valve spool. Thereby, the pressure in the return line and in the pilot line increases again, until the pilot pressure relieve valve opens again, in order to shift the throttle valve spool and lowering the flow rate cross-section, i.e. the throttle cross section. This procedure occurs periodically as long as the pressure in the return line upstream of the throttle valve is higher as the maximum pressure given for the pilot relieve valve which should not be exceeded downstream of the throttle valve. The pilot volumetric flow rate under pilot pressure used to actuate the pilot pressure relieve valve is conducted to an area with lower pressure, e.g. to a tank and withdraws hydraulic fluid from the hydrostatic drive disclosed in DE 10 2004 030 045 B3. In opposite, reversed flow direction and, correspondingly high pilot pressure, the throttle valve spool closes equally and a check valve arranged in parallel is opened such that the hydraulic fluid flow rate is not throttled. However, hydraulic fluid volume gets lost for the working circuit as the pilot pressure relieve valve arranged in the pilot line opens by exceeding continuously the given pilot pressure and, in this manner, takes out a pilot hydraulic fluid flow of the working circuit. This hydraulic fluid flow is absent for the hydrostatic drive, whereby the hydrostatic drive according to DE 10 2004 030 045 B3 cannot transform the whole hydraulic power generated by the hydraulic pump into mechanical power.
In both systems the maximum permissible break power/drag power available at the driving motor is not used optimal as further consumers connected with the drive motor are not considered with regard to its power consumption. Likewise other power losses, the driving machine is subject to, as friction losses, rolling and air resistance are not taken into account in the known decelerating methods. In the known methods systems for hydrostatic deceleration of a hydrostatic drive, the pressure in the return line downstream of the throttle valve—denoted in the following as support pressure—is limited fixedly to a value by the pilot pressure relieve valve. In the known state of the art this maximum support pressure is specifically and unchangeable determined for a driving motor of a work machine. For protection of the work machine and the machine operator under consideration of all possible operation conditions, a work machine can adapt, the support pressure may be chosen to safety reasons only such high that only such an amount of hydraulic power is maximally fed to the hydraulic pump as the driving motor in the coasting mode can received, if no further consumer is coupled to the driving motor, in order that no over speed or damaging on its own of the driving motor or the working machine may occur in any coasting operation condition, in particular not, if no (auxiliary-) consumer power or power losses lowers the revolution of the driving motor. Hence, the predetermined support pressure has to be adapted due to safety reasons to the maximum permissible break power/drag power of the driving motor without taking into account further consumers or auxiliary drives, which can be active in a drag condition as well. However, typically, the driving motor propulses further consumer or auxiliary drives of a work machine which also consume power in a coasting operation. This power consumption being present in a coasting operation of the hydrostatic drive additionally, not only is unconsidered in the known deceleration systems, since furthermore has to be converted into heat power in order to reach the desired deceleration, likewise the exceeding hydraulic power which cannot be decelerated mechanically. At the same time, the driving motor according to the methods known from the state of the art will not reach their maximum permissible drag revolutions in drag operations, if the presented power of (auxiliary-) consumers is bigger than zero and which power of the (auxiliary-) consumers are lowering the revolutions of the driving motor, therefore, the known deceleration methods do not exploit optimally the hydraulic power supportable at the driving motor, in particular not if the power of (auxiliary-) consumers is bigger than zero.
In case of the solution known from the state of the art according to DE 10 2004 030 045 B3 it can be seen additionally that this system generates vibrations as the pilot pressure relieve valve used for controlling of the throttle cross section, is opened and closed repeatedly during the coasting operation, such that the throttle valve spool changes permanently its relative position and opens and closes the flow cross section in an oscillating way.
Furthermore, in the state of the art according to DE 199 30 997 B4, it is proposed to bypass the throttle/the throttle valve by a bypass line for the reverse mode, i.e. during reversed flow direction in the hydraulic circuit, such that the throttle valve does not form a flow resistance for the reverse mode. Herefor, a check valve is provided in the bypass line opening only if the hydraulic line forms a supply line for the hydraulic motor. Such a design as exemplarily shown in FIG. 1 of DE 199 30 997 B4 is bulky, complex, needs valuable construction space and causes high costs.